It is well known in the electrical packaging arts that certain electrical components, for example, electrolytic capacitors, electrochemical cells, batteries of electrochemical cells, and double layer capacitors, can produce gases during continued operation. The pressure of those gases can rupture or burst a package. In order to avoid undue pressure increases within packages containing such components, it is desirable that gases generated within the package escape.
Packages incorporating membranes or plugs that respond to excessive internal pressures are known. Those packages may incorporate a membrane that ruptures or a plug that fuses or is expelled from a package in response to excessive internal pressure. These "one time" pressure relief mechanisms are extremely undesirable in many applications. Once a membrane is ruptured or a plug is melted or expelled, the electrical component within a package is exposed to the ambient. Essential fluids within the package can escape and undesired fluids, such as oxygen and water, can enter the package to cause or accelerate corrosion or performance degradation mechanisms.
A porous diaphragm may also be used to release internal pressure. In U.S. Pat. No. 3,524,112, an electrolytic capacitor incorporating a membrane of rubber or neoprene having a diameter of 1 to 5 millimeters and a thickness of 0.5 to 2 millimeters is described. The membrane permits hydrogen diffusion out of the capacitor while preventing escape of the electrolyte. That patent describes an electrolytic capacitor casing having a groove. A polycarbonate foil having a thickness of 0.02 millimeters is wrapped around the casing over the groove. When pressure within the capacitor becomes too large, an edge of the foil is temporarily displaced from the package surface to allow gas to escape. In addition, the foil permits hydrogen diffusion from within the capacitor. The displacement of the foil means that foreign matter can enter and electrolyte can escape from the package.
A battery vent employing a membrane that is permeable to oxygen and hydrogen but that is impermeable to the sulfuric acid electrolyte is described in U.S. Pat. No. 3,909,302. The membrane, having a thickness of 0.1 to 0.8 millimeters, is microporous polytetrafluoroethylene. The membrane holds back the liquid battery electrolyte while permitting the diffusion of gases out of the battery so long as the liquid electrolyte does not occlude the membrane.
Double layer capacitors employ electrolytic elements comprising activated carbon and an electrolyte, typically sulfuric acid. The carbon contains many pores, producing a very large surface area for charge storage. Two of these electrolytic elements are brought together with an intervening ion/electron selective membrane to form a capacitor element. For increased operating voltage, many of the capacitor elements are stacked with intervening electrically conductive plates as terminals of the individual capacitor elements. See U.S. Pat. No. 4,683,516, the disclosure of which is incorporated herein by reference.
Double layer capacitors have an extremely high energy storage density. Capacitances of one farad and more with essentially unlimited voltage capabilities can be produced by connecting double layer capacitor elements in parallel and series. It is now known that double layer capacitors can generate a gas during operation. Little attention has been given to that gas generation and, to the inventor's knowledge, no one has previously attempted to identify the gas generated. The inventor has learned from chromatographic analysis that the gas generated is carbon dioxide. At least some of the carbon dioxide is believed to be reactively produced from oxygen that has been absorbed on the carbon in the capacitor elements and from oxygen that may leak into the package. Some carbon dioxide generation can be prevented or limited by employing highly purified materials in a double layer capacitor, but the extra cost of purification makes such capacitors unreasonably expensive. In view of the identification of the generated gas as carbon dioxide, venting of carbon dioxide and exclusion of external oxygen from a packaged double layer capacitor has been recognized by the inventor as an important packaging goal.
Even though the total amount of carbon dioxide generated in a double layer capacitor is relatively small, the small volume of a typical double layer capacitor package that is not occupied by solid materials, i.e., the "empty volume", means that relatively large pressures can result from the generated gas. For example, a package having an "empty volume" of one cubic centimeter may be able to withstand an internal pressure of ten atmospheres without bursting. If the packaged capacitor has a design lifetime of ten years, the average gas generation rate cannot exceed about 3X10.sup.31 6 moles per year without bursting the package before the end of its design lifetime. Extremely high purity materials are required to avoid exceeding that tiny gas generation rate.
In double layer capacitors, as in other electrical components that generate gases, a displaceable, fusible, or rupturable plug or a similar pressure-relief mechanism that gives access, even temporarily, to the inside of the package is highly undesirable. That access permits the undesirable entry of deleterious materials into the package and allows useful materials, i.e., the electrolyte, to escape, seriously degrading performance. In double layer capacitors, electrolyte loss, measured by the decrease in the weight of the packaged component, causes a catastrophic decline in capacitance and increase in equivalent series resistance (ESR). (The ESR is a measure of the degree of difficulty of charging and discharging a capacitor. Since a high ESR means a capacitor has failed, ESR is a particularly sensitive indicator of a capacitor's condition.)
In the typical double layer capacitor, a stack of capacitor elements is placed under a compressive force to maintain good electrical contact in the stack. For example, double layer capacitor packages typically employ concave end caps crimped to another part of the package to apply a compressive force to a capacitor element stack. If gas pressure within a double layer capacitor package grows without relief, the double layer capacitor package, including a concave end cap, may be distorted, changing the compressive force applied to the stack of double layer capacitor elements and reducing performance. This source of performance reduction over time, i.e., package distortion and "unloading" of the compressive force on a stack of capacitance elements, has not previously been appreciated and no solution has been proposed.
Accordingly, it is desirable to provide a package for an electrical component that generates a gas during operation that permits the generated gas to escape from the package, that retains desired fluids within the package, and that prevents undesirable materials from entering the package. Most preferably, the package, by selectively venting the gas generated within it, eliminates any necessity for special purification of the constituents of the packaged electrical component that would increase the cost of the packaged component and prevents package distortion so that a stable compressive force is applied to the electrical component within the package during its design lifetime.